This invention relates to linearity compensation used to reduce distortion in amplifiers. More particularly, the invention relates to feedforward techniques in broadband amplifiers employing similar types of amplifying devices.
Various distortion compensation techniques have been devised for improving the performance of amplifiers. Among them are feedforward and feedback. Conventional feedforward arrangements require duplication of power amplifiers, each in a signal path that are combined at the output. The amplifier duplication wastes power while the loss incurred by combining signals at the output reduces the effective output of the amplifier. Feedback, on the other hand, reduces distortion at the expense of gain often requiring further stages of amplification to produce the desired output.
In some applications more than others, such as satellite repeaters, these penalties are serious drawbacks. Of chief concern are compact circuits with low power consumption and minimal weight for use in satellites. Another concern is that the circuits in satellites are typically operated at high levels to obtain high output levels for the total power consumed which unfortunately tends to exacerbate the effect of nonlinearities.
Due to the ever increasing demand in the amount of information to be transmitted, the need for linear amplifiers with wide bandpass characteristics is highly desirable. Currently, transponders operating in designated bands in a satellite repeater are required to prevent intermodulation distortion of multipath interference between received signals being amplified and redirected toward earth stations. Such an approach is costly in terms of the loss of bandwidth efficiency due to the required spacing between designated bands and the weight of the several sections in the filter demultiplexers used in such an approach.
Another linearization technique is known as predistortion compensation wherein predistortion components are added to the input signal to provide destructive interference between the predistortion components and a selected order of the distortion introduced by the amplifier while in the process of amplifying. Conventional arrangements, however, typically utilize passive circuitry (e.g., diode arrangements) in the predistorter which reduces distortion by partially emulating the distortion characteristics of the active amplifier. In effect, a large portion of the input signal is required to provide sufficient signal levels for operating the predistorter before amplification to generate a particular order of distortion components. As a result, the gain or amplification factor is reduced to improve performance for the selected order of distortion.
Although these drawbacks are particularly acute in satellite applications, an amplifier which overcomes such problems will offer attendant advantages that are highly beneficial in a variety of other applications wherein high linearity and wide bandpass characteristics are desirable. For example, typical applications may include microwave radio and single sideband transmission systems.